1. Introduction
The purpose of this memo is to document how some of the requirements
specified in [RFC1264] for advancing protocols developed by working
groups within the IETF Routing Area to Draft Standard have been
satisfied by LDP. Specifically, this report documents operational
experience with LDP, requirement 5 of section 5.0 in RFC 1264.
LDP was originally published as [RFC3036] in January 2001. It was
produced by the MPLS Working Group of the IETF and was jointly
authored by Loa Andersson, Paul Doolan, Nancy Feldman, Andre
Fredette, and Bob Thomas. It has since been obsoleted by [RFC5036].
2. Operational Experience
This section discusses operational experience with the protocol. The
information is based on a survey sent to the MPLS Working Group in
October 2004. The questionnaire can be found in the MPLS Working
Group mail archives for October 2004.
11 responses were received, all but 2 requesting confidentiality.
The survey results are summarized to maintain confidentiality. The
networks surveyed span different geographic locations: US, Europe,
and Asia. Both academic and commercial networks responded to the
survey.
2.1. Environment and Duration
The size of the deployments ranges from less than 20 Label Switching
Routers (LSRs) to over 1000 LSRs. Eight out of the 11 deployments
use LDP in the edge and the core, two on the edge only, and one in
the core only.
Sessions exist to peers discovered via both the basic and the
extended discovery mechanisms. In half the cases, more than one
adjacency (and as many as four adjacencies) are maintained per
session. The average number of LDP sessions on an LSR ranges from
under 10 to just over 80. The responses are spread out as follows:
under 10: 4 responses, 20-50: 4 responses, and over 80: 1 response.
In the surveyed networks, the time LDP has been deployed ranges from
under 1 year to over 4 years. The responses are spread out as
follows: under 1 year: 3 responses, 2 years: 2 responses, 3 years: 3
responses, and over 4 years: 3 responses.

2.2. Applications and Motivation
Nine of the 11 responses list Layer 3 Virtual Private Networks
(L3VPNs) as the application driving the LDP deployment in the
network.
The list of applications is as follows: L3VPNs: 9, pseudowires: 4
current (and one planned deployment), L2VPNs: 4, forwarding based on
labels: 2, and BGP-free core: 1.
There are two major options for label distribution protocols, LDP and
Resource Reservation Protocol-Traffic Engineering (RSVP-TE). One of
the key differences between the two is that RSVP-TE has support for
traffic engineering, while LDP does not. The reasons cited for
picking LDP as the label distribution protocol are:
o The deployment does not require traffic engineering - 6
o Inter-operability concerns if a different protocol is used - 5
o Equipment vendor only supports LDP - 5
o Ease of configuration - 4
o Ease of management - 3
o Scalability concerns with other protocols - 3
o Required for a service offering of the service provider - 1
2.3. Protocol Features
All deployments surveyed use the Downstream Unsolicited Label
Distribution mode. All but one deployment use Liberal Label
retention (one uses conservative).
LSP setup is established with both independent and Ordered Control.
Five of the deployments use both control modes in the same network.
The number of LDP Forwarding Equivalence Classes (FECs) advertised
and LDP routes installed falls in one of two categories: 1) roughly
the same as the number of LSRs in the network and 2) roughly the same
as the number of IGP routes in the network. Of the 8 responses that
were received, 6 were in the first category and 2 in the second.

2.4. Security Concerns
A security concern was raised by one of the operators with respect to
the lack of a mechanism for securing LDP Hellos.
2.5. Implementations and Inter-Operability
Eight of the 11 responses state that more than one implementation
(and as many as four different ones) are deployed in the same
network.
The consensus is that although implementations differ, no inter-
operability issues exist. The challenges listed by providers running
multiple implementations are:
o Different flexibility in picking for which FECs to advertise
labels.
o Different flexibility in setting transport and LDP router-id
addresses.
o Different default utilization of LDP labels for traffic
resolution. Some vendors use LDP for both VPN and IPv4 traffic
forwarding, while other vendors allow only VPN traffic to
resolve via LDP. The challenge is to restrict the utilization
of LDP labels to VPN traffic in a mixed-vendor environment.
o Understanding the differences in the implementations.
2.6. Operational Experience
In general, operators reported stable implementations and steady
improvement in resiliency to failure and convergence times over the
years. Some operators reported that no issues were found with the
protocol since deploying.
The operational issues reported fall in three categories:
1. Configuration issues. Both the session and adjacency endpoints
must be allowed by the firewall filters. Misconfiguration of
the filters causes sessions to drop (if already established) or
not to establish.
2. Vendor bugs. These include traffic blackholing, unnecessary
label withdrawals and changes, session resets, and problems
migrating from older versions of the technology. Most reports
stated that the problems reported occurred in early versions of
the implementations.

3. Protocol issues.
- The synchronization required between LDP and the IGP was
listed as the main protocol issue. Two issues were
reported: 1) slow convergence, due to the fact that LDP
convergence time is tied to the IGP convergence time, and 2)
traffic blackholing on a link-up event. When an interface
comes up, the LDP session may come up slower than the IGP
session. This results in dropping MPLS traffic for a link-
up event (not a failure but a restoration). This issue is
described in more detail in [LDP-SYNC].
- Silent failures. Failure not being propagated to the head
end of the LSP when setting up LSPs using independent
control.
3. Security Considerations
This document is a survey of experiences from deployment of LDP
implementations; it does not specify any protocol behavior. Thus,
security issues introduced by the document are not discussed.
4. Acknowledgments
The editors would like to thank the operators who participated in the
survey for their valuable input: Shane Amante, Niclas Comstedt, Bruno
Decraene, Mourad Kaddache, Kam Lee Yap, Lei Wang, and Otto Kreiter.
Not all who participated are listed here, due to confidentiality
requests. Those listed have given their consent.
Also, a big thank you to Scott Bradner, who acted as an independent
third party ensuring anonymity of the responses.
The editors would like to thank Rajiv Papneja, Halit Ustundag, and
Loa Andersson for their input to the survey questionnaire.

Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.